1. Field of the Invention
The invention refers to the field of ceramics and relates to an assembled honeycomb, such as can be used, e.g., as a catalytic converter for cleaning exhaust gases, as catalyst substrate and/or as a filter, and as a heat exchanger and light-weight element.
2. Discussion of Background Information
Honeycombs of ceramic materials have been used for many years in environmental technology and the automotive industry for treating and cleaning exhaust gases. The honeycombs are commonly composed of a plurality of elongated channels with different cross-sectional shapes, which channels are separated from one another by thin ceramic webs. Honeycombs of this type are preferably shaped by extrusion (e.g., extrusion molding) of a plasticized ceramic mass through a die, then dried and fired (e.g., sintered). The shape of the channels and the outer shape of the honeycombs are established by the design of the extruder die. In principle, an endless strand is formed by extrusion. A honeycomb is then formed by cutting off vertically or at an angle to the direction of extrusion, which honeycomb has, e.g., prismatic outer contours; the base area of the prism (e.g., triangular, rectangular, square, hexagonal, etc.) is determined by the contours of the die, as stated above.
Depending on the choice of ceramic mass and the subsequent treatment steps, in terms of materials the honeycomb can be composed of different ceramic materials, e.g., aluminum oxide, Cordierite, mullite, titanium oxide, aluminum titanate, silicon carbide, silicon-silicon carbide, silicon nitride, aluminum nitride, carbon and various ceramets. It is also possible with this method to extrude and sinter masses of metallic powders and to therewith produce metallic honeycombs, e.g., of high-temperature-resistant alloys, such as Ni-based alloys or FeCrAl or FeCrAlY. The porosity of the materials can differ in quantity (e.g., pore volume), form and size. For applications as a catalytic converter, special catalyst materials are extruded, e.g., TiO2 with WO3 and V2O5, for the NOx reduction of flue gases and exhaust gases (e.g., so-called DeNOx catalytic converters) or hexaaluminates for high-temperature combustion. For applications as a catalyst substrate, the channel walls of the honeycombs are subsequently coated with washcoats to increase surface, in which washcoats catalytically active substances, e.g., noble metals such as Pt, are embedded.
For exhaust gas cleaning of, e.g., solvent-containing exhaust gases from paint shops, the honeycombs are coated with adsorbents, e.g., zeolites or with activated carbon. Lithium silicates, for example, can be used for the adsorption of CO2 from combustion gases. The harmful constituents are removed from the exhaust gas and concentrated in the regeneration gas through alternating flow through with exhaust gas and with a gas for regeneration. Heat exchangers work in a similar manner, in which the honeycombs are first heated with a hot gas and subsequently, by switching over to a cold gas, the cold gas is heated up while flowing through.
In some applications, the channel walls of the honeycombs are made from a material with open, i.e., continuous porosity, and the channels are alternately closed several millimeters deep on both end faces such that each channel is closed on only one side and channels closed on one end face are adjacent to channels that are not closed (see, e.g., U.S. Pat. No. 4,329,162). In this manner a gas stream is forced to flow through the porous channel walls, whereby particles are filtered out of the exhaust gas stream and gaseous constituents are very effectively cleaned by the above-mentioned catalyst coatings. Honeycombs of this type have been used very successfully in recent years as diesel particulate filters.
In the above-mentioned technical applications monolithic honeycombs are rarely used for various reasons, instead several individual honeycombs are connected (by adhesion, sticking or clamping) to form an assembled honeycomb (see, e.g., U.S. Pat. No. 4,304,585) by joining to one another the lateral prism walls lying parallel to the flow direction. With respect to the assembled honeycomb, the individual honeycombs from which it is constructed are referred to below as honeycomb segments. These honeycomb segments are likewise composed of a plurality of elongated channels of different cross-sectional shapes that are separated from one another by thin ceramic webs. The individual channels of the different honeycomb segments lie parallel to one another in the connected honeycomb.
There are various reasons for assembling honeycombs instead of using monolithic structures. For example, to produce large honeycomb cross sections, a correspondingly large die and extruder cross section are necessary. This, however, is technically complex and causes great difficulties during burning (sintering) of the honeycombs, such that it is easier to instead assemble individual honeycomb segments after burning to form one large body.
Also, temperature gradients occur in the honeycomb with many technical applications, which, depending on the coefficient of expansion and thermal conductivity of the wall material, leads to thermomechanical stresses that can cause a distortion or the occurrence of cracks in the honeycomb. The relief of such stresses in a honeycomb is likewise achieved by using segmentations. If the honeycomb segments lie loosely against one another and are held only by their shape, e.g., by an external tensile force, each segment can expand and move virtually freely, so that the thermal stresses at the interface are completely relieved. However, it is often necessary to connect the individual segments to one another by adhesive force in order to achieve a higher overall strength of the assembled honeycomb or to ensure the joints are leak-proof with respect to the exhaust gas to be cleaned. This is achieved by gluing or sticking the honeycomb segments to the above-referenced prism walls, whereby the connecting layers should have a defined thickness and a lower modulus of elasticity than the material of the honeycomb segments. Moreover, the adhesive layers usually have a lower strength than the honeycomb segment material, so that in the event of excessive stresses, the adhesion joints are more likely to tear than the honeycomb segment material, so that the functionality of the assembled honeycomb is initially maintained.
It is understood that the geometry of the honeycomb segments influences the outer geometry of the assembled honeycomb. This, in turn, determines whether the body achieves the outer geometry required for the respective technical application, or whether the required outer geometry has to be produced by a laborious reworking procedure.
However, the size and geometry of the honeycomb segments also considerably influence properties that affect the production and application of the honeycombs. Thus, for example, the size and the outer shape of the honeycomb segments influence their mechanical strength depending on the load direction. With thermal load, the geometry of the honeycomb segments has a strong impact on the temperature distribution in the segments and in the assembled honeycomb, and on the mechanical stresses occurring due to the thermal expansion
Through the combination of the honeycomb segments to form an assembled honeycomb, contact surfaces and contact edges are formed in the interior of the assembled honeycomb. Seam lines and contact points of the sides or corners of the cross sections of the individual segments result in the cross section of the assembled honeycomb perpendicular to the flow direction.
The geometry of the honeycomb segments thereby has a strong impact on the number, size and position of the contact areas and contact edges located in the interior of the assembled honeycomb. The geometric shape of the honeycomb segments therefore has a great impact on the mechanical stability of the assembled honeycomb which affects the processing and use of the honeycombs.
These segmentations are used to a large extent with diesel particulate filters of silicon carbide (e.g., European Pat. Appln. Nos. EP 0 816 065, EP 1 142 619). Typically, individual honeycomb segments with a square cross section (with rounded corners) are glued to one another to form a large block. Special gluing geometries and gluing materials are designed to achieve improved durability (e.g., European Pat. Appln. No. EP 1 291 061, and International Pub. Nos. WO 2005/084782, WO 2005/071234).
When honeycomb segments with square cross section are used, as described in European Pat. Appln. No. EP 1 508 356, in the cross section, 4 contact points are formed at the corners of the segments, which is relatively advantageous. The stability of the individual segments under mechanical stress perpendicular to the flow direction is relatively good and unfavorable only in the direction of the cross-sectional diagonal. Straight continuous seam lines running at a 90° angle to one another are necessary in such applications. A very high waste of at least 20% occurs in the production of round outer contour cross sections.
When honeycomb segments with the cross section of an equilateral triangle are used, as likewise described in EP 1 508 356, they have a very high strength under mechanical stress perpendicular to the flow direction, in particular under stress on the edges. The glued seams in the cross section of the assembled honeycomb perpendicular to the flow direction advantageously run at an angle of 60° to one another but form many straight continuous seam lines that are very unfavorable for the strength of the assembled honeycomb. It is particularly unfavorable in terms of strength that the segments in the interior of the assembled honeycomb meet at respectively 6 edges or in cross section at 6 corners. The cross section of the assembled honeycomb perpendicular to the flow direction can be designed as an equilateral hexagon or as an elongated hexagon, such that less waste occurs when producing round or oval outer-contour cross sections compared to the use of honeycomb segments with a square cross section.
When honeycomb segments are used with the cross section of an equilateral regular hexagon, as likewise described in EP 1 508 356, they have a low strength under mechanical stress perpendicular to the flow direction, in particular under stress on the edges. The glued seams in the cross section of the assembled honeycomb perpendicular to the flow direction advantageously run at an angle of 120° to one another and do not form straight continuous seam lines that are unfavorable for the strength of the assembled honeycombs. It is also advantageous for strength that only 3 segments meet at the edges or in cross section at 3 corners. However, it is very unfavorable that the outer cross section of the assembled honeycomb has many concave outer contours. Due to this, the assembled honeycombs cannot be sealed and installed without processing, and a high degree of waste occurs in the production of round or oval outer-contour cross sections.
With other conventional assembled honeycombs, the honeycomb segments have a circle-segment cross section, so that honeycombs with round cross-sectional geometries can be produced from these segments without reworking. However, with a high number of segmentations and large cross-section diameters of the honeycombs, the segments have a very elongated and acute-angled cross-sectional geometry, which is unfavorable for the strength of the individual segments. Moreover, the edges of all of the segments meet in the interior of the honeycomb at one point, which is unfavorable for the strength of the honeycomb. Therefore, with large cross-sectional diameters of assembled honeycombs, a centric honeycomb with a round cross section is used and around it several satellite honeycomb segments surrounding it are attached, which segments in cross section have the shape of annular segments. However, at least two different segment shapes are necessary for this and the mechanical stability of the concave side of the satellite honeycomb segments is unfavorable.